1. Field of the Invention
The present invention relates to a seat belt retractor for retracting a seat belt of a vehicle.
2. Description of the Related Art
When a vehicle receives an impact during a collision or the like, a retractor for retracting a webbing (a belt) of a seat belt apparatus is locked so that the webbing cannot be unreeled any further unless it is pulled with considerable force. Meanwhile, a pretensioner provided in the retractor is also activated to retract the webbing, thereby eliminating looseness in the webbing, before an upper body of an occupant tilts forward. Thus, the webbing contacts a body of the occupant closely, and as a result, a restraining performance of the seat belt is improved (Japanese Patent Application Publication No. 2009-173103, for example).
When the upper body of the occupant is tilted forward by the impact of the collision so that a bodyweight of the occupant acts on the webbing with a force that exceeds the aforementioned considerable force, a load limiter mechanism of the retractor is activated to unreel the webbing by a small amount. Thus, a load applied to the body of the occupant from the webbing is lightened, and as a result, the occupant is protected from injuries such as a broken rib. When the load limiter mechanism is activated, the upper body of the occupant tilts forward to a certain extent (see Japanese Patent Application Publication No. 2004-210005, for example).
The forward-tilting upper body of the occupant is also supported by an air bag activated when the collision occurs, and therefore the occupant is protected such that a head portion of the occupant does not collide with a frontward steering wheel or the like (see Japanese Patent Application Publication No. 2009-012661, for example).
A seat belt apparatus must satisfy predetermined legal requirements. According to recent regulations pertaining to collision safety, a certain degree of safety must be secured regardless of a physique of the occupant. According to U.S Federal Motor Vehicle Safety Standards (FMVSS) in particular, safety requirements during a crash test vary between a fifth percentile adult female (AF 5%) and a fiftieth percentile adult male (AM 50%). Safety requirements can be cleared by applying a small restraining force to the former, but cannot be cleared unless at least an equivalent restraining force is applied to the latter.
An adaptive load limiter (LLA) has been proposed to satisfy these requirements. An adaptive load limiter includes a mechanism for switching the load applied to the body of the occupant from the webbing on the basis of a build of the occupant, which is detected by an occupant detection sensor.
However, a vehicle may receive an impact from two consecutive collisions. For example, a first impact may be received when the vehicle drives over a curbstone separating a road from a sidewalk and a second impact may be received when the vehicle continues to travel until it collides with a guardrail. However, activation leeway may remain in the load limiter mechanism after being activated by the first impact. In this case, the load limiter mechanism is reactivated when the bodyweight of the occupant is exerted on the webbing by the second impact, and as a result, the body of the occupant may tilt further forward.
The pretensioner of the retractor and the air bag, on the other hand, are activated by the first impact and cannot be reactivated. Therefore, at the time of the second impact, the load limiter mechanism is activated in a state where the pretensioner and the air bag are not functional. When the upper body of the occupant is tilted forward by the load limiter mechanism in this state, the occupant may be exposed to danger.
Further, an adaptive load limiter that satisfies predetermined regulations requires the occupant detection sensor in order to detect the build of the occupant and the switching mechanism for switching the load, and therefore a cost increase occurs.